Drivetrain Vibration

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desert_max

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Hey guys, I have an issue that came to light today on my first extended cruise up into the mountains. A group of us did it 330 mile loop with some heavy usage - significant elevation change (1500 to 7000ft!) and good curvy mountain roads. Bike (86) ran flawlessly, but is oozing brake fluid out the lid of my newly rebuilt front brake master cylinder, and blowing oil out of a cracked PVC hose in the valley. Those are minor little annoyances.

What is concerning me is a vibration that I had somewhat detected before, but really felt today. It’s not the typical handlebar vibration that folks complain about. This one is most prevalent under load headed up a steep grade. It’s a cyclical, rhythmic vibration (pulses like once a second) that I feel in my seat not the handlebars. Does that sound like driveshaft to anyone? I plan on tearing into it, but was hoping somebody might recognize the symptoms.

Thanks in advance for any musings.
 
The final drive is pretty well bullet proof but I have read of the middle gear output shaft shearing at the securing nut.
I'd look there first.
 
Sean has mentioned about the driveshaft failing on heavily-modified bikes. His take is that it's better to shear a driveshaft than to break an engine case. My friend at the shop where I hang-out, who has built power-adder VMaxes of all-types, said that he had to go to a hardened driveshaft when one of the bikes he built kept breaking them. I think he said it was an Exactrep part.

Seeing as your bike is likely stock, does the vibration go-away when you pull-in the clutch? How-about when in neutral, and you rev the bike? Is it there, or not? That could help you to isolate the location of the source. That pop-out plug in the swingarm, remove that, and see if you have any slop in the driveshaft. Use something like a socket extension, or a pair of long needle-nose pliers to grab the driveshaft, and wiggle it, to see if it's sloppy.

Grab a wheel/tire at the top, and try to push it and pull it sideways, to see if you have worn wheel bearings.

If you have the clutch cover off, see if the basket has any up & down slop in it. Same for the clutch hub, which might require removing the 'steelies' and friction discs.
 
As noted above, the problem only manifests itself when the drivetrain is under load.

I hadn't thought about the clutch basket or pack, but at this point am leaning strongly towards a driveshaft and/or Universal issue.

Bottom line, I think I'm due for some exploratory surgery. Bike only has 23000 miles on it, so I'd be really surprised if something is horribly wrong. But you never know...

Thanks for the ideas.
 
Could be as simple as a bad rear tire. My 98 did that under hard acceleration. Turned out to be the rear tire. I lived it it for a while and then one day when the bike was being dyno tuned it was very obvious when the bike was just idling on the dyno that it was moving up and down in the rear. The tire either had a high or low spot in it.

Changed out the tire and it went away.
 
Could be as simple as a bad rear tire. My 98 did that under hard acceleration. Turned out to be the rear tire. I lived it it for a while and then one day when the bike was being dyno tuned it was very obvious when the bike was just idling on the dyno that it was moving up and down in the rear. The tire either had a high or low spot in it.

Changed out the tire and it went away.

Yes, a good suggestion. Does the rate of vibration increase with speed? That reminds me, and very important, you could have a tread separation happening, and that could cause your tire to fail suddenly and catastrophically. I have a pic of an acquaintance's rear tire having one, the outside tread was bulging, and causing a rhythmic vibration. On dismounting the tire, the inner casing had a very-visible bulge into which air was moving, and which caused the ply separation. A name-brand tire, not a cheapo, and not even 1/4 worn.

IMG_0307.JPG IMG_0306.JPG
 
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Two year old Dunflop Qualifier. But you can bet your hiney that, based on those comments, I'm going to scrutinize it closely!

And this vibration, while fairly prominent, doesn't terrify me, it does concern me though. Definitely need to find out what it is. I had not contemplated the possibility of a catastrophic failure, but certainly cannot discount the possibility.
 
Coincidentally, the tire which failed with probably 1,000 miles on it, on a H-D big twin, was a Dunlop. I searched on-here for a pic of it, I thought I'd posted 2 pics before, but I couldn't find 'em. It looked like a goiter on the outside of the tire, actually on the tread. Inside, it's a fissure in the underside of the tread, very-noticeable.

Update: found the pics, above.
 
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Once I get the ole girl back on the road, I will poke at this a bit more. I don't think it's tire related. Frequency is too high. I will see.

And by back on the road I mean after straightening out the PVC routing debaucle, the superbike bar installation and the addition of DW Holeshot exhaust...currently out for ceramic coating. (Thanks Kyle).
 
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